1. Field of the Invention
The present invention generally relates to hermetically sealed electromagnetic (EM) windows and fabrication method.
2. Description of the Related Art
Radio frequency (RF) windows are used to provide a vacuum seal for RF devices, such as an RF amplifier and a low loss, low reflection transmission path for the RF energy entering and exiting the device. The RF device is typically provided with RF input and output ports with hermetically sealed RF windows, allowing the device to be vacuum sealed. In the case of an amplifier, an electron gun is used whose heater element will oxidize and degrade the amplifier's performance unless it is vacuum sealed.
Hughes Aircraft Company produces Traveling Wave Tubes (TWTs) that incorporate RF windows. One such TWT is the 8910H disclosed in a May, 1989 product sheet. The RF window is presently formed by inserting a dielectric insert into a waveguide, and brazing the insert to the waveguide's inner walls to form a hermetical seal. The dielectric is preferably a ceramic such as aluminum oxide Al.sub.2 0.sub.3 or beryllium oxide (BeO), which have good RF and mechanical properties and relatively low dielectric losses. FIG. 1 is a plot 10 of the insert's absorption losses (T.sub.E) versus frequency for a particular RF window. The insert provides a low loss passband 12 or "window" between approximately 8 and 10 GHz, and a high loss stopband 14 outside that frequency range. The insert's material and thickness are chosen to provide the desired RF passband for a given waveguide shape, typically rectangular or circular.
FIG. 2 is a perspective view of the RF window 16 produced by Hughes. The window comprises a unitary rectangular waveguide 18 and a dielectric insert 20. The dielectric insert 20 is positioned in the waveguide 18 such that its peripheral edge 22 contacts the waveguide's inner walls 24. The technique used to form a hermetic seal is illustrated in FIGS. 3a and 3b. As shown in FIG. 3a, the waveguide is positioned upright and a wire loop 26 of brazing material is inserted through the top of the waveguide and positioned along an edge 28 that is formed by the waveguide's inner walls and the insert. As shown in FIG. 3b, when the waveguide is heated the brazing material liquifies and reflows between the insert's peripheral edge and the waveguide's inner walls to form a hermetic seal 30.
The insert's size and shape must match the inner dimensions of the waveguide to a very exact tolerance, typically at most 25.4.mu.m. If the insert is too big, it will not fit. Conversely, if the insert is too small, the hermetic seal will be weak. Hence, machining the waveguide and the insert is a very slow, complicated and expensive process.
The insert must be positioned in the waveguide at a predetermined location such that it is perpendicular to the waveguide's inner walls and parallel to the end of the waveguide. If the insert is positioned incorrectly the hermetic seal will be weakened and the electrical characteristics of the window will be degraded. A specially made tool (not shown) is used to hold the insert in the correct position until the seal is formed. The tool is expensive, wears out relatively quickly, is tedious to use and holds the insert only in approximately the desired position.
The window is suitably heated up to approximately 1000.degree. C. for several hours to form the hermetic seal. Waveguides are preferably light weight Kovar.RTM. tubes that are lined with copper, although they may be solid copper. The waveguides have a thermal coefficient of expansion of approximately twice that of the ceramic insert at the brazing temperature. During brazing the waveguide tries to expand away from the insert, thus weakening the hermetic seal. To mitigate this problem, the tool is also used to squeeze the waveguide and prevent its expansion. If the pressure on the waveguide is too low, the waveguide will expand and weaken the seal. If the pressure is too high, the liquified material will not be able to flow between the insert's peripheral edge and the waveguide's inner walls, which will also weaken the seal. To further reduce the stress on the hermetic seal, the waveguide's walls are very thin so that they can flex as the temperature changes. However, the thinner walls weaken the waveguide.
The RF window and the process for constructing the hermetic seal are very slow and expensive because of the tight machining tolerances that are required, the use of the specially made tool, and the unequal thermal expansion between the waveguide and the insert. Furthermore, the RF windows have only approximately a 50% yield rate.